The invention is directed to a control system for ohmic loads, preferably signal lamps in a road traffic system, in which a frequency generator controls test switches associated with the individual ohmic loads, with the test switches being operatively connected at the output side to an evaluation switch, operative to control the supply of current to such evaluation switch.
The control of the "red" or "stop" lights of traffic signal systems is of particular importance with respect to safety, as substantially incorrect operation of such lights can result in serious accidents. Consequently, it is not sufficient to merely monitor such "red" lights as to their individual correct functioning, but to provide an adequate safety factor in the overall control of the system and to effect suitable control thereof in the event of an improper functioning of one of the "red" lights. It is known to provide test switches with a test frequency and to use the absence of the test frequency as a criterion for the occurrence of faults. As transistors are generally utilized as test switches, there is always the possibility and risk that a short circuit may take place whereby the operating voltage falsely indicates the presence of the test frequency, and thus the operating fault remains undetected. In order to prevent this situation, it has heretofore been suggested to provide a transformer as a decoupling member between the test switch and the evalution switch, but such transformer has a technically high production cost and in addition the size thereof presents a disadvantage, particularly in view of general miniaturization techniques being employed at the present time.
The invention therefore is directed to the problem of providing a circuit in which the effect of operational faults are always reflected by a desired actuation of the evaluation switch, i.e. always toward the safe side, and in which the circuit arrangement is smaller in size and can be technically produced with less effort than previously described circuitry.